CYP21A2 and hyperinsulinemic hypoglycemia, familial, 4: The most common enzymes are 21-hydroxylase (CYP21A2), 11β-hydroxylase (CYP11B1) and 17α-hydroxylase (CYP17A1).(1,2) The first corresponds to more than 90% of cases, presenting 20%-50% of residual enzymatic activity in its nonclassic form (NCAH).(1,3) This Position Statement will preferably address NCAH resulting from 21-hydroxylase deficiency, the most frequent and important differential diagnosis of polycystic ovary syndrome (PCOS).